Conventional fine silver particle colloidal dispersions are those in which fine particles of a noble metal such as silver have been dispersed in a solvent in a low concentration of 10% by weight at most, and are used as a transparent conductive layer forming coating fluid used to prevent electromagnetic waves from leaking in computer displays or the like (see Japanese Patent Applications Laid-open No. H11-329071 and No. 2000-268639), a coating fluid for forming antimicrobial coatings (see Japanese Patent Application Laid-open No. H4-321628), and so forth.
For example, in the use for preventing electromagnetic waves from leaking in computer displays or the like, a front glass panel of a cathode ray tube (CRT) is coated with the transparent conductive layer forming coating fluid by spin coating or the like, followed by drying and thereafter baking at a temperature of about 200° C. to form a transparent conductive layer.
Recently, however, in order to form conductive silver layers or the like more simply according to uses, a method is also proposed in which a fine silver particle colloidal dispersion paste in which fine silver particles have been dispersed in a high concentration is used to carry out printing by a printing method such as screen printing, followed by baking at a temperature of about 200° C. (see Japanese Patent Application Laid-open No. 2000-334618).
However, the fine silver particle colloidal dispersion paste used for such a purpose has ever been produced by an evaporation-in-gas method in which silver is evaporated and condensed in a gas under reduced pressure to collect the silver in a solution containing a polymeric dispersing agent. Hence, this method affords a very poor productivity, and therefore the fine silver particle colloidal dispersion paste thus obtained has been very expensive. Moreover, in this fine silver particle colloidal dispersion paste, a polymeric dispersing agent or the like capable of combining strongly with the surfaces of fine silver particles is contained in order to improve dispersion stability, and hence it is necessary to decompose and remove it by high-temperature heat treatment after printing and drying. This can not be said to be preferable.
A method is also proposed in which fine silver particles are formed by wet-process reaction to produce a fine silver particle colloidal dispersion in which the fine silver particles have been dispersed in a high concentration (see Japanese Patent Application Laid-open No. 2003-103158). This method, however, makes use of a polymeric dispersing agent capable of combining strongly with the surfaces of fine silver particles, in order to improve the stability of dispersion of the fine silver particles formed at the time of the reaction. Accordingly, where a conductive silver film is formed using this fine silver particle colloidal dispersion in which the fine silver particles have been dispersed in a high concentration, there has been a problem that the conductive silver film to be formed can not easily be made to have a low resistivity if heat-treated at a low temperature.
In addition, in respect of fine silver particle colloidal dispersions such as the fine silver particle colloidal dispersion paste, the viscosity of fine silver particle colloidal dispersions is required to be freely controllable, in order to make them adaptable to various printing or coating methods such as screen printing, gravure printing and ink-jet printing. Such viscosity is commonly controlled by adding a viscosity modifying component such as a binder resin to a system. However, like the above polymeric dispersing agent, the viscosity modifying component added is required to be heat-treated at a high temperature, and such a requirement has caused a problem that the conductive silver film formed has a poor conductivity.